cpython/Lib/weakref.py

"""Weak reference support for Python.

This module is an implementation of PEP 205:

http://www.python.org/dev/peps/pep-0205/
"""

# Naming convention: Variables named "wr" are weak reference objects;
# they are called this instead of "ref" to avoid name collisions with
# the module-global ref() function imported from _weakref.

import collections

from _weakref import (
     getweakrefcount,
     getweakrefs,
     ref,
     proxy,
     CallableProxyType,
     ProxyType,
     ReferenceType)

from _weakrefset import WeakSet

ProxyTypes = (ProxyType, CallableProxyType)

__all__ = ["ref", "proxy", "getweakrefcount", "getweakrefs",
           "WeakKeyDictionary", "ReferenceType", "ProxyType",
           "CallableProxyType", "ProxyTypes", "WeakValueDictionary",
           "WeakSet"]


class WeakValueDictionary(collections.MutableMapping):
    """Mapping class that references values weakly.

    Entries in the dictionary will be discarded when no strong
    reference to the value exists anymore
    """
    # We inherit the constructor without worrying about the input
    # dictionary; since it uses our .update() method, we get the right
    # checks (if the other dictionary is a WeakValueDictionary,
    # objects are unwrapped on the way out, and we always wrap on the
    # way in).

    def __init__(self, *args, **kw):
        def remove(wr, selfref=ref(self)):
            self = selfref()
            if self is not None:
                del self.data[wr.key]
        self._remove = remove
        self.data = d = {}
        d.update(*args, **kw)

    def __getitem__(self, key):
        o = self.data[key]()
        if o is None:
            raise KeyError(key)
        else:
            return o

    def __delitem__(self, key):
        del self.data[key]

    def __len__(self):
        return sum(wr() is not None for wr in self.data.values())

    def __contains__(self, key):
        try:
            o = self.data[key]()
        except KeyError:
            return False
        return o is not None

    def __repr__(self):
        return "<WeakValueDictionary at %s>" % id(self)

    def __setitem__(self, key, value):
        self.data[key] = KeyedRef(value, self._remove, key)

    def copy(self):
        new = WeakValueDictionary()
        for key, wr in self.data.items():
            o = wr()
            if o is not None:
                new[key] = o
        return new

    def get(self, key, default=None):
        try:
            wr = self.data[key]
        except KeyError:
            return default
        else:
            o = wr()
            if o is None:
                # This should only happen
                return default
            else:
                return o

    def items(self):
        L = []
        for key, wr in self.data.items():
            o = wr()
            if o is not None:
                L.append((key, o))
        return L

    def items(self):
        for wr in self.data.values():
            value = wr()
            if value is not None:
                yield wr.key, value

    def keys(self):
        return iter(self.data.keys())

    def __iter__(self):
        return iter(self.data.keys())

    def itervaluerefs(self):
        """Return an iterator that yields the weak references to the values.

        The references are not guaranteed to be 'live' at the time
        they are used, so the result of calling the references needs
        to be checked before being used.  This can be used to avoid
        creating references that will cause the garbage collector to
        keep the values around longer than needed.

        """
        return self.data.values()

    def values(self):
        for wr in self.data.values():
            obj = wr()
            if obj is not None:
                yield obj

    def popitem(self):
        while 1:
            key, wr = self.data.popitem()
            o = wr()
            if o is not None:
                return key, o

    def pop(self, key, *args):
        try:
            o = self.data.pop(key)()
        except KeyError:
            if args:
                return args[0]
            raise
        if o is None:
            raise KeyError(key)
        else:
            return o

    def setdefault(self, key, default=None):
        try:
            wr = self.data[key]
        except KeyError:
            self.data[key] = KeyedRef(default, self._remove, key)
            return default
        else:
            return wr()

    def update(self, dict=None, **kwargs):
        d = self.data
        if dict is not None:
            if not hasattr(dict, "items"):
                dict = type({})(dict)
            for key, o in dict.items():
                d[key] = KeyedRef(o, self._remove, key)
        if len(kwargs):
            self.update(kwargs)

    def valuerefs(self):
        """Return a list of weak references to the values.

        The references are not guaranteed to be 'live' at the time
        they are used, so the result of calling the references needs
        to be checked before being used.  This can be used to avoid
        creating references that will cause the garbage collector to
        keep the values around longer than needed.

        """
        return self.data.values()


class KeyedRef(ref):
    """Specialized reference that includes a key corresponding to the value.

    This is used in the WeakValueDictionary to avoid having to create
    a function object for each key stored in the mapping.  A shared
    callback object can use the 'key' attribute of a KeyedRef instead
    of getting a reference to the key from an enclosing scope.

    """

    __slots__ = "key",

    def __new__(type, ob, callback, key):
        self = ref.__new__(type, ob, callback)
        self.key = key
        return self

    def __init__(self, ob, callback, key):
        super().__init__(ob, callback)


class WeakKeyDictionary(collections.MutableMapping):
    """ Mapping class that references keys weakly.

    Entries in the dictionary will be discarded when there is no
    longer a strong reference to the key. This can be used to
    associate additional data with an object owned by other parts of
    an application without adding attributes to those objects. This
    can be especially useful with objects that override attribute
    accesses.
    """

    def __init__(self, dict=None):
        self.data = {}
        def remove(k, selfref=ref(self)):
            self = selfref()
            if self is not None:
                del self.data[k]
        self._remove = remove
        if dict is not None: self.update(dict)

    def __delitem__(self, key):
        del self.data[ref(key)]

    def __getitem__(self, key):
        return self.data[ref(key)]

    def __len__(self):
        return len(self.data)

    def __repr__(self):
        return "<WeakKeyDictionary at %s>" % id(self)

    def __setitem__(self, key, value):
        self.data[ref(key, self._remove)] = value

    def copy(self):
        new = WeakKeyDictionary()
        for key, value in self.data.items():
            o = key()
            if o is not None:
                new[o] = value
        return new

    def get(self, key, default=None):
        return self.data.get(ref(key),default)

    def __contains__(self, key):
        try:
            wr = ref(key)
        except TypeError:
            return 0
        return wr in self.data

    def items(self):
        for wr, value in self.data.items():
            key = wr()
            if key is not None:
                yield key, value

    def keyrefs(self):
        """Return an iterator that yields the weak references to the keys.

        The references are not guaranteed to be 'live' at the time
        they are used, so the result of calling the references needs
        to be checked before being used.  This can be used to avoid
        creating references that will cause the garbage collector to
        keep the keys around longer than needed.

        """
        return self.data.keys()

    def keys(self):
        for wr in self.data.keys():
            obj = wr()
            if obj is not None:
                yield obj

    def __iter__(self):
        return iter(self.keys())

    def values(self):
        return iter(self.data.values())

    def keyrefs(self):
        """Return a list of weak references to the keys.

        The references are not guaranteed to be 'live' at the time
        they are used, so the result of calling the references needs
        to be checked before being used.  This can be used to avoid
        creating references that will cause the garbage collector to
        keep the keys around longer than needed.

        """
        return self.data.keys()

    def popitem(self):
        while 1:
            key, value = self.data.popitem()
            o = key()
            if o is not None:
                return o, value

    def pop(self, key, *args):
        return self.data.pop(ref(key), *args)

    def setdefault(self, key, default=None):
        return self.data.setdefault(ref(key, self._remove),default)

    def update(self, dict=None, **kwargs):
        d = self.data
        if dict is not None:
            if not hasattr(dict, "items"):
                dict = type({})(dict)
            for key, value in dict.items():
                d[ref(key, self._remove)] = value
        if len(kwargs):
            self.update(kwargs)